The conversion of coal to liquid and gaseous fuel products is becoming of ever increasing importance in view of the vast reserves of coal in the world compared to the stores of liquid petroleum. Thermal techniques for the conversion of coal to liquid and ashless coal products is being studied, and proposals are under consideration for the catalytic conversion of coal to liquid type products. The catalytic conversion of coal, however, requires very rugged catalysts in view of the high ash, high sulfur and high metals content of the coal charge stocks. In addition, the liquefaction of coal tends to produce considerable amounts of coke, which normally weaken the catalyst physically during required regenerations of the catalyst to restore the activity of the catalyst to a reasonable level.
Research efforts have been directed, therefore, to the finding of a supported catalyst containing a hydrogenation component for use in the liquefaction of coal which possesses reduced coking tendencies and improved physical strength characteristics, especially after repeated regenerations to remove coke and/or metals deposits.
In accordance with the invention, an improved catalyst has been discovered for use in a process for the liquefaction of coal in a reaction zone in the presence of hydrogen under coal liquefaction conditions. The improved catalyst comprises a hydrogenation component distributed substantially uniformly on a support comprising an amorphous aluminum phosphate.
The solid carbonaceous materials that can be used herein can have the following composition on the moisture-free basis:
______________________________________ Weight Percent Broad Range Normal Range ______________________________________ Carbon 45 - 95 60 - 92 Hydrogen 2.5 - 7.0 4.0 - 6.0 Oxygen 2.0 - 45 3.0 - 25 Nitrogen 0.75 - 2.5 0.75 - 2.5 Sulfur 0.3 - 10 0.5 - 6.0 ______________________________________
The carbon hydrogen content of the carbonaceous material will reside primarily in benzene compounds, multi-ring aromatic compounds, heterocyclic compounds, etc. Oxygen and nitrogen are believed to be present primarily in chemical combination with the aromatic compounds. Some of the sulfur is believed to be present in chemical combination with the aromatic compounds and some in chemical combination with inorganic elements associated therewith, for example, iron and calcium.
In addition to the above, the solid carbonaceous material being treated herein may also contain solid, primarily inorganic, compounds which will not be convertible to liquid product herein, which are termed as "ash", and are composed chiefly of compounds of silicon, aluminum, iron and calcium, with smaller amounts of compounds of magnesium, titanium, sodium and potassium. The ash content of a carbonaceous material treated herein will amount to less than 50 weight percent, based on the weight of the moisture-free carbonaceous material, but in general will amount to about 0.1 to about 30 weight percent, usually about 0.5 to about 20 weight percent.
Anthracitic, bituminous and subbituminous coal, lignitic materials, and other types of coal products referred to in ASTM D-388 are exemplary of the solid carbonaceous materials which can be treated in accordance with the process of the present invention to produce upgraded products therefrom. When a raw coal is employed in the process of the invention, most efficient results are obtained when the coal has a dry fixed carbon content which does not exceed 86 percent and a dry volatile matter content of at least 14 percent by weight as determined on an ash-free basis. The coal, prior to use in the process of the invention, is preferably ground in a suitable attrition machine, such as a hammermill, to a size such that at least 50 percent of the coal will pass through a 40-mesh (U.S. Series) sieve. The ground coal is then dissolved or slurried in a suitable solvent. If desired, the solid carbonaceous material can be treated, prior to reaction herein, using any conventional means known in the art, to remove therefrom any materials forming a part thereof that will not be converted to liquid herein under the conditions of reaction.
Any liquid compound, or mixtures of such compounds, having hydrogen transfer properties can be used as solvent herein. However, liquid aromatic hydrocarbons are preferred. By "hydrogen transfer properties" we mean that such compound can, under the conditions of reaction herein, absorb or otherwise take on hydrogen and also release the same. A solvent found particularly useful as a startup solvent is anthracene oil defined in Chamber's Technical Dictionary, MacMillan, Great Britan 1943, page 40, as follows: "A coal-tar fraction boiling above 518.degree. F. [270.degree. C.], consisting of anthracene, phenanthrene, chrysene, carbazole and other hydrocarbon oils." Other solvents which can be satisfactorily employed are those which are commonly used in the Pott-Broche process. Examples of these are polynuclear aromatic hydrocarbons such as naphthalene and chrysene and their hydrogenated products such as tetralin (tetrahydronaphthalene), decalin, etc., or one or more of the foregoing in admixture with a phenolic compound such as phenol or cresol.
The selection of a specific solvent when the process of the present invention is initiated is not critical since a liquid fraction which is obtained during the defined conversion process serves as a particularly good solvent for the solid carbonaceous material. The liquid fraction which is useful as a solvent for the solid carbonaceous material, particularly coal, and which is formed during the process, is produced in a quantity which is more than sufficient to replace any solvent that is converted to other products or which is lost during the process. Thus, a portion of the liquid product which is formed in the process of the invention advantageously recycled to the beginning of the process. It will be recognized that as the process continues, the solvent used initially becomes increasingly diluted with recycle solvent until the solvent used initially is no longer distinguishable from the recycle solvent. If the process is operated on a semicontinuous basis, the solvent which is employed at the beginning of each new period may be that which has been obtained from a previous operation. For example, liquids produced from coal in accordance with the present invention are aromatic and generally have a boiling range of about 300.degree. to about 1400.degree. F. (149.degree. to 760.degree. C.), a density of about 0.9 to about 1.1, and a carbon to hydrogen mole ratio in the range of about 1.3:1 to about 0.66:1. A solvent oil obtained from a subbituminous coal, such as Wyoming-Montana coal, comprises a middle oil having a typical boiling range of about 375.degree. to about 675.degree. F. (191.degree. to about 357.degree. C.). Thus, the solvent that is employed herein can broadly be defined as that obtained from a previous conversion of a carbonaceous solid material in accordance with the process defined herein. Although the term "solvent" is used, it is understood that such term covers the liquid wherein the liquid product obtained herein is dissolved as well as the liquid in which the solid materials are dispersed.
The ratio of solvent to solid carbonaceous material can be varied so long as a sufficient amount of solvent is employed to effect dissolution of substantially all of the solid carbonaceous material in the reaction vessel. While the weight ratio of solvent to solid carbonaceous material can be within the range of about 0.6:1 to about 4:1, a range of about 1:1 to about 3:1 is preferred. Best results are obtained when the weight ratio of solvent to solid carbonaceous material is about 2:1. Ratios of solvent to solid carbonaceous material greater than about 4:1 can be used but provide little significant functional advantage in dissolving or slurrying the solid cabonaceous material for use in the process of this invention. An excessive amount of solvent is undesirable in that added energy or work is required for subsequent separation of the solvent from the system.
The catalyst for use in the process of this invention comprises a support material and a hydrogenation component substantially uniformly distributed over the surface of the support material. The hydrogenation component can be any hydrogenation catalyst well known to those having ordinary skill in the art, but preferably the hydrogenation component comprises at least one hydrogenating component selected from the group consisting of metals, metal oxides and/or metal sulfides of Groups VI and/or VIII of the periodic table. More preferably the hydrogenation component comprises at least one hydrogenating component selected from the group consisting of the metals, metal sulfides and/or metal oxides of (a) a combination of about 2 to about 25 percent (preferably about 4 to about 16 percent) by weight molybdenum and at least two iron group metals where the iron group metals are present in such amounts that the atomic ratio of each iron group metal with respect to molybdenum is less than about 0.4 and (b) a combination of about 5 to about 40 percent (preferably about 10 to about 25 percent) by weight of nickel and tungsten where the atomic ratio of tungsten to nickel is about 1:0.1 to 5 (preferably about 1:0.3 to about 4 ), said hydrogenating component being composited with a porous support. Particularly preferred among the hydrogenating metals are nickel, cobalt, molybdenum and tungsten. Catalysts of type "(a)" may contain molybdenum in the amounts conventionally used, i.e., about 2 to about 25 percent molybdenum based on the total weight of the catalyst including the porous carrier. Smaller amounts of molybdenum than about 2 percent may be used, but this reduces the activity. Larger amounts than about 25 percent can also be used but do not increase the activity and constitute an extra expense. It is preferred to utilize a catalyst containing about 4 to about 16 percent by weight molybdenum, most preferably about 10 percent; about 2 to about 10 percent by weight nickel, most preferably about 2 percent; and about 1 to about 5 percent by weight cobalt, most preferably about 1.5 percent. While a three-metal component catalyst as in "(a)" is preferred, a two-metal component catalyst as in "(b)" can also be used. When using a two-metal component catalyst, it is preferred to utilize one containing about 15 to about 25 percent (e.g., about 19 percent) tungsten and about 2 to about 10 percent (e.g., about 6 percent) nickel supported on a porous carrier such as alumina. In a two-metal component catalyst, the weight ratio of tungsten to nickel is preferably in the range of about 2:1 to about 4:1 tungsten to nickel, respectively. The amounts of the iron group metals in "(a)" and "(b)" may be varied as long as the above proportions are used. However, in "(a)" it is preferred to utilize one iron group metal in an atomic ratio between about 0.1 and about 0.2 and to use the other iron group metal or metals in an atomic ratio of iron group metal to molybdenum of less than about 0.l and especially between about 0.05 and about 0.1. All of the iron group metals may be present but it is preferred to use only two. The amount of hydrogenating component based on the metal itself can suitably be from about 0.5 to about 60 percent by weight of the catalyst including the porous carrier, but is usually within the range of about 2 to about 30 percent by weight of the catalyst including the carrier.
The above-mentioned active hydrogenating components can also be present as mixtures. On the other hand, chemical combinations of the iron group metal oxides or sulfides with the molybdenum oxide and/or sulfide can be utilized.
The catalytic hydrogenation components discussed above either singly or in combination, are deposited substantially uniformly on a support comprising amorphous aluminum phosphate. The amorphous aluminum phosphate can be used alone. Preferably the aluminum phosphate is an amorphous coprecipitate containing aluminum, phosphorus and oxygen; and while these elements are chemically combined, their precise chemical connection is not certain. X-ray diffraction patterns indicate that the aluminum phosphate material is amorphous. While the aluminum phosphate can be used alone, an amorphous coprecipitate containing excess aluminum, as the oxide, can also be used. Usually the atomic ratio of aluminum to phosphorus in the preferred supports is in the range of about 11:1 to 1:1 and is preferably in the range of 5:1 to 1:1.
Amorphous precipitates of aluminum phosphate are known in the art. These precipitates are prepared by neutralization of a strongly acidic aqueous medium containing aluminum cations and PO.sub.4 .sup.-.sup.-.sup.- anions in a substantially equal molar ratio. Such acidic solutions are prepared by dissolving in water a highly soluble aluminum salt and a source of PO.sub.4 .sup.-.sup.-.sup.- ions, usually ortho-phosphoric acid. The aluminum salt employed is not critical, provided only that it does not contain an anion which will form a precipitate in the subsequent precipitation step. Aluminum nitrate and aluminum halides, particularly aluminum chloride, are the aluminum salts of choice for use in the invention. While certain phosphate salts such as triammonium ortho-phosphate can be used as the source of the PO.sub.4 .sup.-.sup.-.sup.- ions, ortho-phosphoric acid is the source of choice for providing the PO.sub.4 .sup.-.sup.-.sup.- ions.
The amorphous aluminum phosphate precipitate is prepared by neutralizing the acidic medium containing aluminum cations and phosphate anions. When the pH is increased to 6 or higher, the aluminum and phosphorus moieties precipitate from the aqueous medium. While in theory the neutralization can be carried out by mixing the acidic solution with an appropriate alkali in any manner, it is preferred to simultaneously add the acidic medium and the neutralizing alkali to a stirred aqueous medium. The two solutions should be added at controlled rates so that the pH is continuously maintained at a preselected pH in the range of about 6.0- 10.0. While a wide variety of bases can be used to neutralize the acidic medium, it is preferred to use ammonium hydroxide or an ammonium salt such as ammonium carbonate so that the aluminum-phosphorus precipitate will be free of metallic ions that might be incorporated into the precipitate, if inorganic bases such as sodium carbonate or sodium hydroxide were used in the process. While the precipitation reaction can be carried out over a wide range of temperatures, ambient temperature usually is employed, as no significant advantages are obtained by heating or cooling.
After the precipitation is completed, the precipitate is filtered, washed one or more times to free the precipitate of occluded ions, and dried. Thereafter, the precipitate is calcined in a conventional manner at a suitable temperature, typically in a range of about 125.degree.500.degree. C. No advantages are obtained by calcining at higher temperatures, and it is preferred to avoid calcining the product at temperatures above about 1100.degree. C., as some crystallization takes place at these higher temperatures. A product calcined for 4 hours at 1100.degree. C. appeared to be crystalline and to have a rudimentary tridymite-type structure.
The calcined aluminum phosphate product is amorphous, typically has a bulk density in the range of about 0.25 to 0.5 grams/cm.sup.3, and typically has the appearance of a compacted mass of spherical granules having a diameter in the 100-500 micron range.
As noted above, the support for the catalyst of this invention can also be an amorphous coprecipitate containing aluminum and phosphorus moieties in which the aluminum and phosphorus are present in an atomic ratio within the range previously described.
The aluminum-phosphorus containing coprecipitates are prepared by the same procedures employed to prepare the aluminum phosphate precipitates, except that the molar ratio of aluminum cations to PO.sub.4 .sup.-.sup.-.sup.- anions is adjusted to a range from about 11:1 to above 1:1, and preferably a range of 5:1 to above 1:1 and more preferably to a range from about 3.5:1 to about 1.2:1.
As certain aluminum salts, ortho-phosphoric acid and ammonium hydroxide are soluble in certain polar solvents such as methanol, it is possible to prepare the previously described inorganic carriers by carrying out the indicated synthesis steps in such polar solvents or in mixtures of water and such polar solvents.
In addition, the catalyst supports or carriers can contain other materials such as alumina and silica. It has been found that the addition of a separate silica-alumina aids in the pelleting of the aluminum phosphate compositions described above. The silica content of the added silica-alumina can suitably be from 50 to 90 weight percent of the silica-alumina. The silica-alumina added can suitably be present in an amount from 10 to 30 weight percent of the composite catalyst.
The formation of the composite catalysts, i.e. the hydrogentation component plus the support comprising aluminum phosphate, can occur by any method which is well known in the art, and such methods do not per se form a part of the present invention. Such methods include, for example, the deposition of one or more salts of hydrogenation metals onto the preformed support by a suitable technique such as the technique of minimum excess solution (incipient wetness). Other suitable techniques are described, for example, in U.S. Pat. No. 2,880,177, which issued to R. A. Flinn and J. B. McKinley on Mar. 31, 1959.
The catalysts can be formed into any suitable size for use in fluid bed, ebullating bed or fixed-bed operation, and preferably a fixed-bed operation is employed. In a fixed-bed operation, the particle size of the composited catalyst can suitably be from 1/32" diameter to 1/4" diameter extrudate, or about 1/32" to 1/4" diameter spheroids.
When treating a carbonaceous material, such as a coal slurry, according to the process of the invention, it is customary to continue the reaction until the catalyst activity has decreased markedly due to the deposition of ash and/or coke or other carbonaceous material thereon. In the process of the present invention, the reaction will continue over an extended period of time before regeneration of the catalyst is required. When regeneration of the catalyst becomes necessary, the catalyst can be regenerated by combustion, i.e., by contact with an oxygen-containing gas such as air at an elevated temperature, usually about 900.degree. F. (482.degree. C.), or by any other means normally used to regenerate hydrogenation catalysts. The manner in which the catalyst is regenerated does not constitute a portion of the present invention. It is one of the advantages of the catalysts for use in the process of this invention that they retain their physical strength characteristics after repeated oxidative burn-off regenerations and, in fact, surprisingly, appear to increase in strength after an oxidative burn-off regeneration.